Opioids are highly effective in the clinical treatment of pain but sustained administration results in the development of tolerance and may lead to addiction. Even after prolonged abstinence, former addicts can remain vulnerable to relapse, particularly under stressful conditions. Much research on the neurobiological mechanisms of opiate addiction and relapse has focused on opioid regulation of dopaminergic reward pathways. Fewer studies have examined the role of the serotonin (5-hydroxytryptamine;5-HT) system in opiate addiction, particularly at the single cell level. The serotonin system may contribute to the affective components of addiction: both euphoric responses to opiates and dysphoric states during withdrawal. Dysregulation of the 5-HT system by long-term exposure to opiates may also underlie the high rate of comorbidity of affective disorders such as depression with opiate dependence. In addition, the 5-HT system regulates dopaminergic neurotransmission, thus may indirectly play a role in more traditional reward pathways. The objective of this application is to test the hypothesis that the serotonergic dorsal raphe nucleus (DRN) is regulated by opioids and stress and furthermore that this neural circuit is a substrate for stress-induced opiate relapse. This objective will be achieved using a combination of electrophysiological, immunohistochemical, pharmacological and behavioral techniques. AIM 1 will compare the effects of the stress neurohormone corticotropin-releasing factor (CRF) and morphine on GABA- and glutamatergic synaptic activity in 5-HT DRN cells in vitro. Next, the role of DRN circuits in stress-induced morphine relapse will be investigated using electrophysiological (AIM 2) and behavioral (AIM 3) models. These studies will characterize the DRN as an integrator of inputs from multiple neurotransmitter pathways that are engaged by both opioids and stress to impact 5-HT neurotransmission. These studies may also identify novel targets for the treatment of opiate addiction and the prevention of relapse. Opiate addiction and relapse are significant public health concerns. This proposal aims to investigate the neurobiological basis for these conditions from single cell physiology to behavioral levels. This approach may identify novel targets for the treatment of opiate addiction and the prevention of relapse.